Organic materials that can be induced to undergo a shape change have recently attracted a great deal of interest in mechanical actuator applications. These materials convert chemical, electrical, or electromagnetic energy into mechanical work. Electromechanical actuators are of particular interest as synthetic muscle materials. While a few clever approaches and materials have been proposed for emulating the action of muscles, this research area is still in its infancy and synthetic muscles remain an unattained goal of material scientists.
Electromechanical actuators based on redox-active polymers have been the most extensively studied systems for mimicking the action of muscles. The actuation mechanism in these materials is based on bulk volume changes that result from the uptake and expulsion of counterions during the redox cycle as shown in FIG. 1. Since the counterions in these systems have specific volumes, their introduction and removal from a bulk polymer result in the respective increase and decrease of the overall volume of the material. Unfortunately, slow cycle times and limited cycle lifetimes have prohibited the use of redox-active polymers as synthetic muscles. These limitations have fueled the search for new materials that are capable of electromechanical actuation via different mechanisms.
One of the more interesting approaches for achieving electromechanical actuation is based on [8]annulenes, which are eight-membered macrocycles with alternating single-and-double carbon bonds. These systems have tub-like structures in their neutral state that can undergo redox-induced tub-to-planar conformational changes. For example, the parent [8]annulene, cyclooctatetraene, undergoes a reversible conformation change from a contracted structure to a planar structure upon two-electron reduction described in Scheme 1 below. This conformation change also results in an increase
in distance between nonadjacent carbon atoms (e.g., dplanar and dtub Scheme 1) that can be used to mimic the expansion and contraction of muscle tissue.
A useful way of harnessing the tub-to-planar geometry change of [8]annulenes in electomechanical actuators is to incorporate such ring systems into polymer structures, since polymers allow for facile processing into useful shapes. However, for such polymer materials to be useful in these applications they must also exhibit stable and reversible redox chemistry and be conjugated to facilitate long-range redox communication between repeat units. It is also highly desirable that their synthesis be facile and allow for structural variation so that the properties of the materials can be tailored to the needs of specific applications.
It turns out that these requirements place severe limitations on the use of polymers containing [8]annulene units in electromechanical actuation. For example, conjugated polymers incorporating cyclooctatetraene units are not useful in these applications because they are exceedingly difficult to prepare and can only be reduced under inert atmospheric conditions. While the redox stability of conjugated polymers containing [8]annulenes are dramatically improved by fusing six-membered rings to the cyclooctatetraene units such as in 1, 2,5,6-dibenzocyclooctatetraene 1 and tetra benzocyclooctatetraene 2, conjugated polymers incorporating these units have not been reported (presumably because they are also difficult to prepare). Additionally, the steric hindrance of the adjacent phenyl rings in 2 prevents its tub-to-planar conformation change. These restrictions have severely limited the ability to prepare useful polymer materials for electromechanical actuation based on the redox-induced conformational change of [8]annulene units.
